1. Field of the Invention
The present invention relates to a coil apparatus for transmitting/receiving electromagnetic waves and a magnetic resonance imaging apparatus using the same (hereinafter referred to as “MRI apparatus”).
2. Background Art
MRI apparatuses are designed to irradiate an examinee placed in a uniform static magnetic field produced by a magnet with an electromagnetic field, excite nuclear spin in the examinee's body, then receive a nuclear magnetic resonance signal which is electromagnetic wave produced by the nuclear spin and image the examinee. Irradiation of the electromagnetic wave and reception of the nuclear magnetic resonance signal are realized by an RF coil which transmits or receives radio frequency (RF) electromagnetic waves and transmission coils, reception coils or dual-function coils in variety of shapes suitable for MRI apparatuses have been developed.
A signal measured by an MRI apparatus is usually signals from hydrogen nuclei contained in water or fat. In recent years, the magnetic field of such MRI apparatuses has been intensified and sensitivity has been improved. As a result, it is possible to acquire signals of nuclei other than hydrogen (called “signals from other nuclides”). For example, signals from nuclei such as phosphorus, carbon, fluorine and sodium can be acquired. It is expected that acquisition of information on muscle or protein that relates metabolism of will be available by imaging signals of phosphorus and carbon.
MRI signals from other nuclides are generally as extremely weak as on the order of 1/10 to 1/100 or less of an MRI signal of hydrogen, and it is thereby difficult to obtain detailed images. Therefore, it is a general practice to display MRI signals from other nuclides with low resolution superimposed on morphologic images created from the MRI signal of hydrogen. In this case, the MRI signal of hydrogen and MRI signals of other nuclides are preferably acquired simultaneously or successively.
Since the coil used for MRI transmission/reception has high frequency selectivity, the coil to acquire an MRI signal of hydrogen and the coil to acquire signals of other nuclides are normally set independently each other. Attempts to make one type of coil tune two frequencies have also been made.
For example, Patent Document 1 discloses a method of tuning two or more frequencies using an RF coil made up of a plurality of conductors called “rung” arranged in an axial direction between concentrically arranged single cylinders under a scheme of multiple patch resonators. Furthermore, Patent Document 2 discloses a scheme of tuning two or more frequencies by connecting inductor/capacitor resonant circuits in parallel with birdcage type coil rungs. Furthermore, Patent Document 3 discloses a scheme of tuning a birdcage type coil with two or more frequencies by providing a plurality of sets of portion called “end ring.”
[Patent Document 1] U.S. Pat. No. 5,557,247
[Patent Document 2] U.S. Pat. No. 4,916,418
[Patent Document 3] U.S. Pat. No. 6,100,694